The present invention relates to an optical imaging system, and, more particularly, to an all-optical extended focus imaging system including a multiplexed set of Fresnel lenses (FL's), such that the optical imaging system of the present invention produces clear images of objects at a variety of distances from the optical imaging system without requiring mechanical refocusing, although with a sacrifice of contrast and resolution when compared with an image produced by a sharply focused lens or lens system of similar size. Similarly, the present invention reduces the sensitivity of image quality to the distance of the detector from the optical imaging system.
Various attempts have been made to provide optical imaging systems which can produce sufficiently sharp and bright images of objects whose distances from the optical imaging system are variable, or in situations where the distance from the optical imaging system to the detector may not be precisely controlled.
The depth of field (DOF) of an optical imaging system is the range of distances from the optical imaging system at which an object can be placed such that the image of the object will be acceptably well-focused.
It is well known to those skilled in the art that the depth of field of a lens system is increased by reducing the aperture of the lens. However, reduction of the aperture of the lens reduces the brightness of the image. Diffraction effects associated with small apertures also cause blurring of the image.
Automatic focus systems are also well known to the art. However, automatic focus systems take time to focus, add weight and complexity to the optical system, require a source of power, often make unwanted acoustic noise, and sometimes do not focus on the object of interest but instead on some other object. Automatic focus systems also are not adequate in situations where it is desired to obtain an image of more than one object simultaneously, where the objects are not at the same distance from the optical imaging system.
It is well known that imaging systems are sensitive to misfocus, i.e., variations in image or object plane location. See, for example, J. W. Goodman, Introduction to Fourier Optics, McGraw-Hill, New York, 1996, pp.126–151, which is incorporated by reference for all purposes as if fully set forth herein. Many attempts have been made to reduce this sensitivity, in order to obtain an increased depth of field. Some approaches were based on using apodized apertures, meaning absorptive masks in the pupil aperture, as well as stopping the aperture down. In all-optical approaches, an increase in tolerance in the position of the image or object location is coupled with a decrease in optical power throughput, as well as image resolution reduction. Recently, hybrid, opto-digital approaches that overcome these deficiencies by using a non-absorptive phase mask and digital post-processing restoration operations have been demonstrated in the literature. Unlike the all-optical approach of the present invention, the hybrid approaches require an electronic processing stage to follow the optical acquisition.
There is thus a widely recognized need for, and it would be highly advantageous to have, an optical imaging system that produces images of acceptable quality of objects at a wide variety of distances from the optical imaging system without the need to adjust the focus of the optical imaging system.